The researchers conducted four independent experiments in the lab and in the field. They found that enhancing the activity of these fungi by raising CO2 levels actually promoted the decomposition and release of organic carbon from the soil.

The carbon effects seen in this study are also likely to be affected by fertiliser use, says Hu.

"Carbon and nitrogen cycles interact, particularly in agro-ecosystems and forage lands where arbuscular mycorrhizal fungi dominate and human perturbations are routine. In these systems, nitrogen fertiliser inputs are high and some of them are emitted as greenhouse gases too."

To clearly understand the responses of terrestrial carbon cycling to climate change, interactions between carbon and nitrogen cycles must be taken into account, writes George A. Kowalchuk in a separate article in Science.

The decomposition seen in the study "presumably results from the activation of soil-borne microbes by the increased flux of [fungus]-derived substrates," says Kowalchuk, a microbial ecologist from the Netherlands Institute of Ecology.

"This allows the newly stimulated soil microbes to degrade other organic matter sources in the soil."

While the study may undermine some assumptions about plant-soil systems as potential carbon sinks, all is not lost, says Kowalchuk.

"Through manipulations of soil nitrogen, organic matter quality, and field management, the balance of soil decomposition patterns can be tipped in a more positive direction," he says.

Important questions

The study was well conducted and raises important questions, says biophysicist Professor Iain Young, of the University of New England, who was not involved in the research.

"How much carbon can the soil sequester? I've always felt very little," Young says.